The present invention relates to a receiver and especially a diversity combining receiver with channel selection capability and, more particularly, to a diversity combining receiver adapted to cophase only those inputs originating at a single selected source, even though weaker or stronger cochannel signals from one or more different sources are present.
Phased arrays are often employed to gain the advantages associated with diversity and beam steering. When used at a receiver a phased array antenna receives a plurality of inputs, each having a distinct phase angle, and the receiver cophases this plurality of inputs to produce a combined output superior to any one of the inputs. By cophasing, the receiver effectively "steers" the receiving array toward the source of the transmission, and the cophaser may also generate phase information which can be used to direct transmission (conjugate phase retransmission) back toward the source. One cophasing arrangement known as the Granlund combiner mixes the combined output with the input of each branch to eliminate or strip the intelligence. This stripped signal is then mixed with each branch input to eliminate the distinctive phase angle associated with the branch. The Granlund combiner reported initially in "Topics in the Design of Antennas for Scatter" by John Granlund, MIT Lincoln Lab Technical Report No. 135, Nov. 23, 1956 functions with any form of modulation.
In certain systems using phased array antennas, it may be necessary to conserve carrier frequencies by reusing them at the same location. In particular, this is anticipated for satellite communication systems in which differently directed beams, occupying a common frequency band, would be received at one antenna array. However, if a phased array were illuminated by a plurality of beams at the same frequency, a conventional diversity receiver could not select among the different sources of transmission since the common frequency signals, referred to herein as cochannel signals, would not be distinguishable. In fact, a Granlund type combiner could not cophase a selected one of these cochannel signals since it would inherently lock-on the strongest of them, rather than any specifically chosen one.
In copending Patent application Ser. No. 468,416 filed by R. E. Langseth et al on May 9, 1974, now U.S. Pat. No. 3,911,364, a diversity combiner is adapted to cophase only those inputs originating at a single selected source, even though weaker or stronger cochannel signals from a different source are present. The successful selection of one cochannel signal is made possible by two factors. Transmission at each source utilizes phase-shift keying (PSK) for intelligence modulation and additionally the carrier is modulated with a pilot tag distinctive for the transmission of each individual source.
The combiner receives and combines the output from each diversity branch, and the combined output is processed to produce a loop signal uniquely identified by the one unique tag corresponding to a selected source. The processing is arranged so that when this loop signal is mixed with each branch input, the upper sideband product of the loop signal and the one selected input contains the phase information associated with the selected input received at that branch, but contains neither pilot tagging nor (due to the PSK format) intelligence modulation. However, any signal received from another source, when similarly mixed with the loop signal, will yield an upper sideband product containing some intelligence modulation or tagging information. Accordingly, narrowband filtering is used to remove products produced by such unselected sources, and the remaining product, having phase information associated with the selected input, is then used as a cophasing signal. This cophasing signal is mixed with the branch input to produce a branch output containing the selected signal cophased with the corresponding signals produced in other branches. The various branch outputs are added to produce the cophased combined output. In this manner, the combiner cophases the signal having one selected tag in the presence of signals having other distinctive tags or no tags at all.
To generate the desire loop signal, the Langseth et al processing circuitry comprises (a) a processor which prepares the combined output for filtering and removes the N-phase PSK modulation from the principal term of the input to the filter; (b) a variable bandpass filter comprising a local oscillator tuned to a frequency N times the selected tag offset and locked by means of a loop consisting of two mixers, a narrow bandpass filter and a limiting amplifier to generate the desired passband and thereby the desired injection signal; and (c) a mixer for combining the injection signal with the combiner output signal mathematically raised to the (N-1) power, as generated by the processor, to produce the desired loop signal.
Although the Langseth et al arrangement represents a considerable advance in the diversity combining receiver art, it is desirable to provide a diversity combining receiver which will function as well, or better, than the Langseth et al arrangement while providing considerably simplified processing circuitry for generating the desired loop signal.